In the graded divisions of Natural History genus and species are fixed names for certain grades. Thus: Vertebrates form a "division"; the next subdivision, e.g., Mammals, Birds, Reptiles, etc., is called a "class"; the next, e.g., Rodents, Carnivora, Ruminants, an "order"; the next, e.g., Rats, Squirrels, Beavers, a "genus"; the next, e.g., Brown rats, Mice, a "species".
Vertebrates (division).
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Mammals, Birds, Reptiles, etc. (class).
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Rodents, Ruminants, Carnivors, etc. (order).
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Rats, Squirrels, Beavers, etc. (genus).
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Brown rats, Mice, etc. (species).